Introduction Protease activated receptor 2 (PAR2) is a G protein coupled receptor responsive to serine proteases, which plays a key role in inflammation and pain reception. Rheumatoid arthritis (RA) patients have up-regulated surface expression of PAR2 in circulating monocytes which correlates with disease activity. We have previously demonstrated that Par2 ^-/- mice are protected from inflammation, bone erosion, and cartilage destruction in a Freund’s Complete Adjuvant (FCA) induced arthritis model. However, it is unclear how PAR2 affects the composition of the myeloid compartment and osteoclastogenesis.

Objectives The aim of this study was to evaluate the impact of loss of PAR2 on the myeloid compartment and osteoclastogenesis.

Methods Bone marrow (BM) from 6–10 week old Par2 ^-/- and control C57BL/6 mice was cultured in pro-osteoclastogenic media for 5 days and the generated mature osteoclasts, tartrate-resistant acid phosphatase positive (TRAP) multinucleated cells, were counted. The resorption potential of these cells was assessed using osteolysis plates and the total osteo-resorption quantified after 7–14 days of culture. BM was also collected for flow cytometric analysis of the haematopoietic cellular composition (with the following markers: CD3, B220, CD11b, Ly6C, Ly6G, CD115, and CD117) and assessed for osteoclast precursors.

Results In vitro osteoclastogenesis revealed an increase in numbers of mature osteoclasts from Par2 ^-/- BM compared to WT, corresponding with increased levels of resorption. Flow cytometry of BM from both WT and Par2 ^-/- mice showed 3 distinct monocyte populations defined by the cell surface expression levels of CD11b and Ly6C. The overall ratio of these populations was not altered in Par2 ^-/- animals.

Conclusions BM from Par2 ^-/- mice has increased osteoclastogenic potential and overall resorptive activity. We propose that this is not due to differences in bone marrow residing osteoclast pre-cursor numbers. This study indicates a potential role for PAR2 in osteoclast differentiation and bone remodelling.

Acknowledgements The research was funded by Medical Research Scotland (MRS) and supported by Astrazeneca.

Disclosure of interest S. McGrath: None declared, L. Hultin Employee of: Astrazeneca, J. Lockhart: None declared, C. Goodyear Consultant for: AstraZeneca